3 ms·

Also, correct me if I'm wrong, but TPM shifts the password input prompt from traditionally LUKS during boot, to your XDM or whatever display manager you're usin

Also, correct me if I'm wrong, but TPM shifts the password input prompt from traditionally LUKS during boot, to your XDM or whatever display manager you're using? And I seem to recall a recent vulnerability where people were able to bypass the display manager login on some graphical desktop Linux distro. I just don't feel comfortable shifting the threat to this part of the OS, because desktop Linux has not had the extensive testing required yet. It reminds me of when I was in school and you could bypass NT4 or Windows 98 logins by mashing the Enter key or doing weird user input combinations. Basically manually fuzzing. If TPM only adds convenience then I'd prefer to continue entering my LUKS password, and my login password, separately.

proto_lambda 3y ago
If you rely only on TPM for key storage, yes, the disk is unlocked automatically and any sufficiently broken userspace application you can get your hands on will let you access it. You can still combine TPM+passphrase/PIN though, at the cost of having to enter it at boot.
- worksonmine 3y ago
  > at the cost of having to enter it at boot Isn't this the entire point of full disk encryption? You mention cost, but what is even the benefit of encryption that's unlocked by just booting?
  - proto_lambda 3y ago
    With properly functioning secure boot and no bugs in the entire software stack, it doesn't matter if the disk is decrypted automatically, since you can't access the system without OS-level authentication. If you tried to replace system files to let you get in anyway, the secure boot measurements would no longer match up and the decryption fails entirely.
    - worksonmine 3y ago
      I use a very long and inconvenient password for LUKS, and a simpler one for login and root. My lock screen is more a convenience in a trusted environment and not security. The TPM only solution sounds like it would require my very long password every time I leave my desk to get coffee.
    - akaiser 3y ago
      Then again, an attacker can read the decryption key from RAM (freeze and remove the modules, then dump the memory on another system) and decrypt the disk offline. So, data on a stolen laptop which has an unprotected TPM (no PIN to boot) can be considered compromised.
      - mratsim 3y ago
        So you use soldered RAM. And the OS provides hardened memory areas that can't be dumped.
      - proto_lambda 3y ago
        There are such things are RAM encryption, but yes, overall it's more fragile from a security perspective than a strong plain passphrase.
    - Talinx 3y ago
      Relying on no bugs in the entire software stack makes the attack surface quite large. If a laptop is stolen the thief can wait sufficiently long for some vulnerability to be discovered somewhere in the stack. With LUKS only the LUKS encryption has to be good and full disk encryption protects the data.
  - yowai 3y ago
    > You mention cost, but what is even the benefit of encryption that's unlocked by just booting? Ideally, your login screen is secure and allows no bypasses into a shell or similar, so you cannot really access any files on the hard drive. And if you modify some system files or boot another operating system to get around this, you are required to know the disk encryption password to get to them.
deleted 3y ago
[deleted]
g_p 3y ago
You could bind your disk unlock to also include a PIN entered at boot-time (TPM+PIN). This gives you the benefit of system integrity verification at time of boot, but also requiring your input to release the keys (and meaning you can't get to the system lock screen without the PIN).
- 542458 3y ago
  Side note, does anybody know why TPM schemes use a numeric PIN for pre-boot unlock rather than a full alphanumeric password? For a given length (say, 8) a password would presumably be more secure than just a PIN. It’s not like this is a phone where you need users to be able to quickly enter the code on a limited input surface.
  - Foxboron 3y ago
    There is no limit to the password. Usinging a 4 digit numeric PIN is just easier to remember, and because of the bruteforce resistance it doesn't decrease the security.
  - alex7734 3y ago
    If you're going to use a long password there is no point to using the TPM as the length of the password itself is enough brute force resistance.
    - SV_BubbleTime 3y ago
      Some you know + something you have/are?
      - alex7734 3y ago
        I don't think it is that much more secure, given that in this case "something you have" is bolted onto the machine you're attempting to protect and that a sufficiently long password provides enough space to make brute forcing impossible anyway. In my opinion all the TPM achieves in this case is ensuring you lose your data if the machine dies (or if some OS update fucks up and doesn't properly ensure the TPM acknowledges the new version as valid). That said it does help against the so called evil maid attacks, given that it would lock itself out if anyone modifies the OS, so if that's part of your threat model then it is useful, I guess.
vladvasiliu 3y ago
> Also, correct me if I'm wrong, but TPM shifts the password input prompt from traditionally LUKS during boot, to your XDM or whatever display manager you're using? It doesn't "shift" it, it's not the same password. It doesn't work the way it does on a mac, where your user password unlocks the drive so you don't have to enter it again to unlock your user session. Assuming you don't have autologon turned on, without this scheme, you'd have to enter two passwords: LUKS first during boot, then your user password in the XDM. With this scheme, you skip the first password. But yeah, if you don't enable any other protection, like TPM+PIN, your PC will boot on its own all the way to the login manager. So if you don't trust your login manager, that's an issue.
ale42 3y ago
Not sure it was ever possible to bypass NT4 logins with whatever key combination... but Windows 98 was definitely doable.

proto_lambda 3y ago

If you rely only on TPM for key storage, yes, the disk is unlocked automatically and any sufficiently broken userspace application you can get your hands on will let you access it. You can still combine TPM+passphrase/PIN though, at the cost of having to enter it at boot.

worksonmine 3y ago
> at the cost of having to enter it at boot Isn't this the entire point of full disk encryption? You mention cost, but what is even the benefit of encryption that's unlocked by just booting?
- proto_lambda 3y ago
  With properly functioning secure boot and no bugs in the entire software stack, it doesn't matter if the disk is decrypted automatically, since you can't access the system without OS-level authentication. If you tried to replace system files to let you get in anyway, the secure boot measurements would no longer match up and the decryption fails entirely.
  - worksonmine 3y ago
    I use a very long and inconvenient password for LUKS, and a simpler one for login and root. My lock screen is more a convenience in a trusted environment and not security. The TPM only solution sounds like it would require my very long password every time I leave my desk to get coffee.
  - akaiser 3y ago
    Then again, an attacker can read the decryption key from RAM (freeze and remove the modules, then dump the memory on another system) and decrypt the disk offline. So, data on a stolen laptop which has an unprotected TPM (no PIN to boot) can be considered compromised.
    - mratsim 3y ago
      So you use soldered RAM. And the OS provides hardened memory areas that can't be dumped.
    - proto_lambda 3y ago
      There are such things are RAM encryption, but yes, overall it's more fragile from a security perspective than a strong plain passphrase.
  - Talinx 3y ago
    Relying on no bugs in the entire software stack makes the attack surface quite large. If a laptop is stolen the thief can wait sufficiently long for some vulnerability to be discovered somewhere in the stack. With LUKS only the LUKS encryption has to be good and full disk encryption protects the data.
- yowai 3y ago
  > You mention cost, but what is even the benefit of encryption that's unlocked by just booting? Ideally, your login screen is secure and allows no bypasses into a shell or similar, so you cannot really access any files on the hard drive. And if you modify some system files or boot another operating system to get around this, you are required to know the disk encryption password to get to them.

deleted 3y ago

[deleted]

g_p 3y ago

You could bind your disk unlock to also include a PIN entered at boot-time (TPM+PIN). This gives you the benefit of system integrity verification at time of boot, but also requiring your input to release the keys (and meaning you can't get to the system lock screen without the PIN).

542458 3y ago
Side note, does anybody know why TPM schemes use a numeric PIN for pre-boot unlock rather than a full alphanumeric password? For a given length (say, 8) a password would presumably be more secure than just a PIN. It’s not like this is a phone where you need users to be able to quickly enter the code on a limited input surface.
- Foxboron 3y ago
  There is no limit to the password. Usinging a 4 digit numeric PIN is just easier to remember, and because of the bruteforce resistance it doesn't decrease the security.
- alex7734 3y ago
  If you're going to use a long password there is no point to using the TPM as the length of the password itself is enough brute force resistance.
  - SV_BubbleTime 3y ago
    Some you know + something you have/are?
    - alex7734 3y ago
      I don't think it is that much more secure, given that in this case "something you have" is bolted onto the machine you're attempting to protect and that a sufficiently long password provides enough space to make brute forcing impossible anyway. In my opinion all the TPM achieves in this case is ensuring you lose your data if the machine dies (or if some OS update fucks up and doesn't properly ensure the TPM acknowledges the new version as valid). That said it does help against the so called evil maid attacks, given that it would lock itself out if anyone modifies the OS, so if that's part of your threat model then it is useful, I guess.

vladvasiliu 3y ago

> Also, correct me if I'm wrong, but TPM shifts the password input prompt from traditionally LUKS during boot, to your XDM or whatever display manager you're using? It doesn't "shift" it, it's not the same password. It doesn't work the way it does on a mac, where your user password unlocks the drive so you don't have to enter it again to unlock your user session. Assuming you don't have autologon turned on, without this scheme, you'd have to enter two passwords: LUKS first during boot, then your user password in the XDM. With this scheme, you skip the first password. But yeah, if you don't enable any other protection, like TPM+PIN, your PC will boot on its own all the way to the login manager. So if you don't trust your login manager, that's an issue.

ale42 3y ago

Not sure it was ever possible to bypass NT4 logins with whatever key combination... but Windows 98 was definitely doable.